Creating a SuiteScript that is triggered by new Customer Message

I'm just getting started with SuiteFlow, but I'm not finding the process as straightforward as the help documentation indicates. I hope someone can give me pointers to move in the right direction!

Issue:
A message is sent from the Communications subtab of the Customer Record.
Customer replies to the message.
Message is logged in the Communication subtab, but the customer service rep who sent the message is not notified.

Proposed Solution:
Create a SuiteScript that is triggered on record change when a new Message is received in the Communications subtab of the Customer record.

When setting up the Workflow, I want the condition to be Customer Message is New or something like that, but I don't see how to do this. So I'm not past the initial setup of the Workflow.